armv8: K3: am654: Introduce FIT generator script

Add a script that is capable of generating a FIT image
source file that combines ATF, SPL(64 bit) and DT.
This combined image is used by R5 SPL and start ATF
on ARMv8 core.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>